Often in communications, it is desirable to know the relative position of a radio with respect to its communication base (also referred to herein as a “base station”, “base unit”, “base radio”, or simply “base”). In one example application, this information is useful in systems related to “presence”. The term presence generally refers to information about a user's ability or willingness to communicate. In the prior art, the concept of using presence in communication systems is often applied in instant messaging systems. Presence is also used in other network communication systems, such as the Microsoft Unified Communication Service. As applied to the field of headsets, typical presence information may include, for example, whether the headset is being worn by the user, the proximity of the user to the base station, other usage information related to the headset, and whether the user desires to be called.
In some applications, the position information required may be an in-proximity or not-in-proximity binary state, namely either a closer proximity or relatively farther proximity, with the threshold between the two states set by the application. The terms “status” and “state” may be used interchangeably herein. One indicator of relative position is received radio signal strength. Often a number is assigned to this strength and is referred to as the received signal strength indication (RSSI). Most manufacturers who report RSSI generally estimate the received signal power at the antenna either by direct measurement, or digital signal processing, and generate RSSI values that are monotonically related to the received signal power. The number is often calibrated to track power linearly and report the value in dBm.
Received signal strength depends on transmit power level, the direct line of sight and reflection path distances between transmitter and receiver, and the phasing of any reflected radio waves received, where the reflected radio waves are referred to as multi-path. As the direct line of sight distance increases for a fixed transmit power, the received amplitude decreases in general, but can vary about in amplitude around this trend due to reflections. In some situations, the direct path is blocked and only reflections are received.
When the direct path is not blocked, relatively large decreases in received signal strength, referred to as signal “nulls” or “fades” can be caused by reflectors. If the reflector is placed so that the signal path from the source to the receiver, bouncing off the reflector, is an even multiple of a half-wavelength different from the path length of the direct wave, the direct and reflected waves can constructively interfere, resulting in a signal “peak” where the received amplitude may be twice as large as the direct path alone. In general, accurate predictions of real situations are difficult, but one can state in general that the actual received signal strength indication can vary by +6 dB to −infinity depending on the reflector configuration for a simple two-path system. When the direct path is blocked, signal nulls can also occur, and generally RSSI will decrease with distance.
Determining a person's relative position to a base station is a useful input to establishing someone's presence. For example, it can indicate that a person is in audio range to hear an alarm, visual range to see a display, or just that they are in their work area as opposed to on-break. This aspect of their presence can be reported back to a monitor, or provided to someone wishing to communicate with that person. In many cases all that is needed is a rough measure of relative position, either the person is within a certain proximity (i.e., in-proximity) or not (i.e., not-in-proximity) from the base station or object of interest. The precise distance defined by an in-proximity state or not-in-proximity state may be varied depending on the particular application.
In the prior art, a variety of means have been used to determine relative position between two radio devices. These techniques have included time-of-flight measurement, which is complex and expensive and so unsuitable to a headset application, or have used signal level in the absence of a noise-reduction technique to be described, using instead other techniques such as long-term averaging, and have tolerated the slow response time and errors inherent in these approaches. Location has commonly also been done using direct geolocation services and reporting, such as an incorporated GPS receiver and reporting to a communications application. This is unsuitable for a headset application due to size and cost issues, as well as industrial design factors.
As described earlier, radio signal strength indication has been used to indicate relative location. The RSSI may have a monotonic non-linear relationship. A calibrated mapping could be made for relative distance versus RSSI. Alternatively a threshold level for RSSI can be set, creating a binary indicator of either in-proximity or not-in-proximity. Other prior art techniques for relative location include the use of GPS, pulse time delay, and triangulation based on access points.
One advantage of RSSI is that it is usually accessible by software in a radio equipped device, being a measured quantity needed for dynamically optimizing radio system operation. No significant processing or additional components are required like GPS, pulse time delay and triangulation.
However, the use of RSSI to determine relative position may be problematic due to signal nulls resulting from multi-path. This can lead to a false not-in-proximity state determination where a not-in-proximity state is determined by the RSSI dropping below some threshold. There also can be signal peaks due to multi-path which occasionally-cause false in-proximity state determinations.
FIG. 1 is a diagram illustrating direct and reflected path lengths for a headset located at a distance from its radio base station. The simplified system shown in FIG. 1 includes a base station 100 that is the source of a radio signal, a headset 102 and a reflecting surface 104. The signal received at headset 102 is the vector sum of a direct-path signal and a reflected-path signal. As shown in FIG. 1, the signal received at headset 102 is the vector sum of the direct-path signal 106 and reflected-path signal 108. When the relative path lengths are such that the direct-path signal 106 and reflected-path signal 108 arrive in phase, the resultant sum is additive and the received signal level is higher than for just the direct path alone. This may result in a signal peak that is not indicative of the distance between base station 100 and headset 102. When the direct and reflected signals arrive out of phase the resultant sum is subtractive and the received signal level is less than for just the direct path alone. This attenuation may result in a signal null that is not indicative of the distance between base station 100 and headset 102.
As a result, there is a need for improved methods and apparatuses for mobile radio ranging relative to its radio base.